The present invention is a method for separating silyl ketene acetals from carbonyl by-products having a terminal olefinic bond and a boiling point similar to that of the silyl ketene acetals. The method employs a platinum catalyst which specifically effects hydrosilation of the carbonyl by-product's terminal olefinic bond by an organohydrosilane. The platinum catalyst is the reaction product of hexachloroplatinic acid and sym-tetramethyldivinyldisiloxane. In a preferred method the silyl ketene acetal is separated from the hydrosilated carbonyl by-product by distillation.
Silyl ketene acetals are useful reactive intermediates for use as coatings and bonding agents. Processes are known for the production of silyl ketene acetals, but these processes often result in the formation of undesirable carbonyl by-products having a terminal olefinic bond and a boiling point similar to that of the silyl ketene acetal. This similarity of boiling points makes it difficult to separate the silyl ketene acetals from the carbonyl by-product. Therefore, the present invention is a method for specifically hydrosilating the terminal olefinic bond of the carbonyl by-product to increase the molecular weight of the carbonyl by-product and thus facilitate its separation from the desired silyl ketene acetal. The inventors have found that a platinum catalyst which is the reaction product of hexachloroplatinic acid and sym-tetramethyldivinyldisiloxane can effect hydrosilation of the terminal olefinic bond of the carbonyl by-product without effecting hydrosilation of the desired silyl ketene acetal.
Ojima et al., J. Organometallic Chem. 111:43-60 (1976), describe a process for preparing silyl ketene acetals using tris(triphenylphosphine)chlororhodium as a catalyst for the hydrosilation of several methyl 2-alkenoates with trialkylsilanes. Ojima et al. observed in addition to the silyl ketene acetals minor quantities of a 1,2-adduct.
Howe et al., J. Organometallic Chem. 208:401-406 (1981), describe two phosphine-rhodium(II) complexes found to be active catalyst for the hydrosilation of olefins including an .alpha.,.beta.-unsaturated ester to form a silyl ketene acetal. The phosphine-rhodium(II) complexes are bis(tris-o-tolylphosphine)dichlororhodium(II) and bis-(tricyclohexylphosphine)dichlororhodium(II).
Revis et al, EPO-219,322, Pub. Apr. 22, 1987, disclose a process for the manufacture of silyl ketene acetals, the process comprising the contacting of methacrylic acid or an ester of methacrylate acid with a hydrogen-containing silicon material in the presence of a catalyst comprising rhodium complexed with inorganic ligands. Revis et al. teach that carbonyl adducts which boil very close to the desired silyl ketene acetal can be reacted with an excess of the hydrogen-containing silicon material to form a higher boiling specie. The present process using a platinum catalyst can provide a faster and more complete conversion of the carbonyl adduct to a higher boiling specie than the process taught by Revis et al. In addition a lower concentration of platinum catalyst in relation to the concentration of the rhodium catalyst taught by Revis et al., is required.
Dinh et al., Co-Pending U.S. Pat. application No. 07/912,433, describe a process for preparing silyl ketene acetal, the process comprises reacting an organohydrosilane with a vinylic compound in the presence of RhCl(di-tert-butylsulfide).sub.2 as a catalyst. Values for carbonyl by-product produced by this process range from about two to nine percent of weight of products formed by the process. The process taught by Dinh et al. is a preferred process for producing a mixture comprising a silyl ketene acetal and a carbonyl by-product for use in the present method.